The control of soldering temperatures is necessary in the soldering of electrical and mechanical connections in order to obtain a proper solder joint. If soldering temperature is too low a cold joint will result which will provide a poor electrical and mechanical connection which may very easily break under the slightest stress. On the other hand, if soldering temperature is too high, damage may result to the electrical or mechanical components from the excessive heat. A desirable soldering range for tin solder is 425.degree. to 525.degree.F.
Normally, the soldering technician judges the proper application of heat to the solder joint by a visual observation of solder flow. He applies or withdraws the heat probe from the solder in order to regulate temperatures.
However, in some manufacturing processes, such as the building of computer processor backplanes, the observation of solder flow between closely spaced wire-wrap terminal pins is hampered. The close centers of these pins preclude the use of traditionally used copper soldering probes. The use of resistance soldering probes has developed for this application. However, when a resistance soldering unit is used, probe temperatures may fluctuate faster than with a standard conduction soldering unit. This requires an even greater control than with a metal probe conduction unit. Control of the soldering temperature becomes to critical to rely on a technician's judgment.
It is desirable to have an automatic control mechanism to regulate soldering probe temperature to within an acceptable range.
An objective of this invention therefore, is to provide an automatic current cut-off to a resistance soldering probe when probe temperature reaches the maximum acceptable soldering temperature.
Another objective is to provide a control for locking out the power to the soldering probe until probe temperature falls to the minimally acceptable soldering temperature and then to reinstate probe current.
A further objective is to have the above mentioned cut-off and locking-out controls adjustable for different operating time periods.
An even further objective of this invention is to provide the control circuit as economically as possible while maintaining control accuracies.